MAYER : MEDUSA. FKOM THE TORTUGAS, FLORIDA. 37 



7 

 4 4 



Tentacles (1) and (7) are situated at the bases of the simple radial canals, 

 while tentacles (2, 2) and (3, 3) are found at the bases of the two bifurcate 

 canals. In addition to these there are the intermediate sets of tentacles (4, 4), 

 (5, 5), and (6, 6) ; and thus the medusa possesses 12 tentacles, each succes- 

 sive pair being 30° apart. It is very remarkable that through a peculiar pro- 

 cess of growth each tentacle bulb is developed into a young medusa which 

 resembles the adult, and is finally set free into the water. Various stages of 

 this process will be seen by an inspection of Figures 141, 142, and 144. The 

 oldest tentacle is the first to be transformed into a new medusa, and the others 

 follow in the order of their age until all of the tentacles have been cast off. 

 The first stage in this process is the development of a hernia-like outgrowth, 

 involving both entoderm and ectoderm, adjacent to and on the centripetal side 

 of each tentacle bulb upon the floor of the sub-umbrella. Soon after this two 

 pointed outgrowths, appear on both sides of each tentacle bulb, and finally 

 develop into new tentacles. These outgrowing tentacles become larger, and 

 soon a still younger pair make their appearance centrifugal to the first, and 

 these are soon followed by two others which lie centripetally from the oldest 

 pair. Before this, however, 4 short canals (the radial canals of the future 

 medusa) develop, and place the gastric cavity of the future proboscis into com- 

 munication with the circular vessel. An opening then appears in the velum 

 of the adult medusa immediately below the proboscis of the future medusa, and 

 this constitutes the velar opening of the new animal. The proboscis becomes 

 cruciform in cross-section, and finally the new medusa is constricted off and 

 becomes free in the condition represented in Figure 142. Here we see that 

 the simple radial canals, the circular canal, the velum, and the oldest tentacle 

 are stolen directly, so to speak, from the parent medusa. The forked canals, 

 proboscis, and younger tentacles are new growths. Even before the outgrowing 

 medusa is detached from the old one, hernia-like outgrowths appear upon the 



